This invention relates to a method for ultrasonic processing of workpieces with a vibratory structure comprising a sonotrode and a converter wherein ultrasonic energy is supplied to the converter by means of an ultrasonic generator which is switched on and off by electrical switch-on and switch-off signals respectively, only for the duration of processing cycles.
This invention also relates to an apparatus for ultrasonic processing of workpieces, which apparatus comprises a vibratory structure having a sonotrode and an electromechanial converter, a generator connected to said converter and adapted to supply ultrasonic energy, means for generating switch-on and switch-off signals for said generator at the start and end respectively of processing cycles, and at least one output means for providing a selected electrical state parameter (P) of said generator.
Methods and apparatuses of this type are used in particular for join two workpieces by spot, rivet, stud or surface welding, wherein at least one of the two workpieces consists of a thermoplastic material. The workpieces are heated locally strongly enough to melt together. The heating of the workpieces is effected in that a sonotrode excited to vibrate mechanically is pressed against at least one of the workpieces. In processing of only one workpiece, these vibrations can also be used to shape or cut the workpiece. Since sonotrodes vibrate as a rule at ultrasonic frequencies of 20 kHz or 35 kHz for example, such processing operations are generally referred to as ultrasonic processing. The main field of use of the described apparatuses lies in the processing of plastics parts, which are used to house electrical devices and to a special degree in the automobile industry, both for body parts such as bumpers and also for interior linings, such as door linings and consoles. The sonotrodes in contact with the workpiece are designed variously, depending on the nature and material of the workpieces to be processed.
In known apparatuses of this kind (DE 4 439 470 C1) the mechanical vibrations are generated by an electromechanical converter, as a rule consisting of a piezoelectric crystal, which is excited from an ultrasonic generator with an electric alternating current circuit. The converter vibrates with a predetermined amplitude at a corresponding ultrasonic resonance frequency, where typical values are e.g. 35 kHz for the frequency and 7 xcexcm for the amplitude. The vibratory structure or resonant unit consisting of the converter, sonotrode and optionally an interposed amplitude transformation piece is so designed that it is also in resonance at this resonance frequency and the working surface associated with the workpiece vibrates with an amplitude of e.g. 10-40 xcexcm.
Spot, rivet and stud welds are as a rule carried out in that the sonotrode is pressed against selected points or on a so-called weld or rivet pin (dome) of the workpiece, which pin is formed on a first workpiece lying on an anvil, projects through a corresponding hole of a second workpiece to be attached to this and is for example 10 mm high and optionally of hollow cylindrical form. In order to avoid lateral escape of the material of the pin becoming soft in the ultrasonic processing and to achieve an optically pleasing welded connection, the sonotrode as a rule has at its working surface to be applied to the weld or rivet pin, a concave domed, ring-shaped heating zone and a ring-shaped contact surface surrounding this, wherein the contact surface lies on the upper workpiece in question at the end of the welding operation and accordingly encircles the weld site with a ring shape which can be circular, rectangular or any other shape.
Among other things, the instant at which the supply of ultrasonic energy to the sonotrode is terminated is significant for the quality of a weld effected in this way. If the energy supply lasts too long, the ring-shaped contact surface can burn into the upper workpiece, which is undesirable for optical and quality reasons. If on the other hand the energy supply is terminated before the contact surface is bearing on the upper side of the workpiece in question, the weld may be incomplete, with the result that the interlocking or force fit produced by the connection is defective.
Corresponding problems arise with other kinds of welding, especially surface welds with the aid of so-called xe2x80x9cenergy directing structuresxe2x80x9d and other ultrasonic processes.
Numerous methods and apparatuses have already become known for ensuring an adequate weld quality and are directed towards finding the right switch-off time for the energy supply but as a whole they are not completely satisfactory. It is known for example to predetermine a precisely defined welding duration or to feed a precisely defined amount of energy to the converter during each operating cycle. Both methods operate inaccurately, since no tolerances can be taken into account, which arise from weld pins of different heights for example. These methods therefore do not always lead to accurate switching off of the supply of the ultrasonic energy when the contact surface of the sonotrode reaches the surface of the workpiece in question. Corresponding problems arise in these of apparatuses in which the distance to be covered by the sonotrode is adjusted to an absolute value depending on the current welding operation or is monitored by means of electrical sensors. Finally is known to associate a sensor in the form of mechanical feeler with each sonotrode, which runs ahead of the sonotrode and is pushed against the force of a spring after application to the surface of the workpiece in question, until it triggers a switch-off signal for the energy supply by means of an electric switch, at the end of a defined path of movement. Such devices do facilitate a defined welding operation and tolerance compensation and thus ensure a high quality of the welded connection. However it is a disadvantage that they require costly manual adjustments, are inflexible in respect of workpieces which are of like structure but made from different materials (e.g. in the production of inner linings covered with different materials for private car doors) and above all an individual feeler has to be associated with every sonotrode present in a processing station, which leads to substantial space problems in the simultaneous processing of a plurality of closely adjacent weld sites.
In addition it is generally known in quality control to monitor constantly various state parameters of the apparatus initially specified, e.g. the ultrasonic power fed to the converter (DE 3429 776 C2). The production of more satisfactory welded connections can however only be ensured inadequately with such quality control.
It is, therefore, an object of this invention of improving the method and the apparatus of the kind above specified by avoiding the demand of additional sensors, feelers and/or other auxiliary means in the region of the sonotrodes.
An further object of this invention is to improve the method and the apparatus above specified such that fault-free welding results and welded workpieces with high quality are achieved.
Yet another object of this invention is to design the method and the apparatus mentioned above such that an exact termination of ultrasonic energy supply to the sonotrode is possible.
A further object of this invention is to reduce the expense of the apparatus mentioned above.
These and other objects are solved by a method and an apparatus being characterized in that the switch-off signal are generated on the basis of at least one state parameter of the generator.
The invention rests on the recognition gained through protracted investigations that various parameters or state parameters within the ultrasonic generator alter measurably when the sonotrode contact surfaces bear on the surface of the workpieces in question. These changes are used according to the invention as a criterion for the termination of the welding operation and to create a switch-off signal terminating the supply of energy. Additional components in the region of the sonotrodes, e.g. for switching off on reaching a predetermined depth, are therefore superfluous. Moreover the invention can be employed with all normally used workpiece materials.
Further advantageous features of the invention will appear from the following description and the claims.